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Immunocontraception of Mammalian Wildlife: Ecological and Immunogenetic Issues

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Immunocontraception of Mammalian Wildlife: Ecological and Immunogenetic Issues REPRODUCTIONREVIEW Immunocontraception of mammalian wildlife: ecological and immunogenetic issues Desmond W Cooper and Elisabeth Larsen Australian and New Zealand Conservation Laboratories, School of Biological, Earth and Environmental Sciences, The University of New South Wales, Sydney, New South Wales 2052, Australia Correspondence should be addressed to D W Cooper; Email: [email protected] Abstract Immunocontraception involves stimulating immune responses against gametes or reproductive hormones thus preventing conception. The method is being developed for the humane control of pest and overabundant populations of mammalian wildlife. This paper examines three fundamental issues associated with its use: (1) the difficulties of obtaining responses to self-antigens, (2) the likely evolution of genetically based non-response to immunocontraceptive agents, and (3) the possible changes in the array of pathogens possessed by the target species after generations of immunocontraception. Our review of the literature demonstrates that the barriers to an effective immunocontraceptive are at present very basic. Should they be overcome, the effects of immunocontraception on the immunogenetic constitution of wildlife populations through the selection for non- responders must be examined. We suggest that the attempt to use the animal’s own immune system to modulate reproduction may be incompatible with the basic biological function of protection against infectious disease. Research programs on mammalian immunocontraception should involve measurement of the heritability of non-response and an assessment of the likely change in the response of the contracepted population to possible pathogens. Reproduction (2006) 132 821–828 Introduction future (Rao 2001, Aitken 2002). Women’s health advocates have objected to all forms of immunocon- The regulation of human and animal population traception because of perceived health risks and the numbers constitutes a difficult and largely unsolved potential for political abuse of the vaccine (Richter contemporary problem. In the developed world, steroid 1994). Human male immunocontraceptives have contraceptives for humans are both widely used and received much less attention and do not appear to be efficacious. Elsewhere they are too costly. The develop- feasible in the near future (Delves et al. 2002). ment of less expensive methods is considered necessary Immunocontraceptives for wild animals have a (Aitken et al. 1993). One such method is immunocon- different objective than those for humans. Their main traception, i.e. the vaccination against sperm, eggs, or aim is to check population growth rather than to reproductive hormones to prevent either fertilization or contracept particular individuals. If some animals are the production of gametes. Attempts to design human irreversibly sterilized so much the better whereas such immunocontraception have a long history (Joshi 1973, an effect in human medicine would be ethically most Stevens 1975, Basten 1988, Gupta & Talwar 1994). The undesirable. Immunocontraceptives for animals are targets include sperm antigens, oocyte antigens, ostensibly humane and could potentially be used on especially zona pellucida proteins (PZP), gonadotropin the large scale required for wildlife population riboflavin carrier protein, gonadotropins and gonado- regulation. Research progress to date has been reviewed tropin releasing hormones (Delves et al. 2002). The most in Tyndale-Biscoe (1991, 1994), Barber & Fayrer-Hosken advanced method involves immunization against human (2000), Barlow (2000), and Cooper & Herbert (2001). chorionic gonadotropin, in reality a method of very early Three fundamental questions remain to be addressed: (1) pregnancy termination (Baird 2000). It now seems likely Can sufficiently strong immune responses be provoked that problems associated with autoimmune disease and against the antigens (immunogens) of gametes or variability of response will prevent any widespread use reproductive hormones to cause contraception in a of immunocontraception in humans in the foreseeable proportion of animals large enough for effective q 2006 Society for Reproduction and Fertility DOI: 10.1530/REP-06-0037 ISSN 1470–1626 (paper) 1741–7899 (online) Online version via www.reproduction-online.org Downloaded from Bioscientifica.com at 09/28/2021 06:24:58AM via free access 822 D W Cooper and E Larsen population management? (2) How rapidly will variation immunizations were boosted at least once (see Table 2). in these responses lead to the evolution of failure to The need for multiple injections and the dependence respond to the immunocontraceptive agent? (3) What upon adjuvant to achieve the necessary level of response will be the ecological consequences of the likely renders the whole approach impractical at present. The changes to the immunogenetic constitution of the most commonly used adjuvant, Freund’s Adjuvant, also population as a result of selection for non-responders? induces a range of undesirable side effects and its use is In particular, will the endemic pathogens of the species being challenged on animal welfare grounds (Leenaars change? There is considerable information which allows et al. 1994, 1998). There is at present no feasible or us to answer at least in part the first two questions. The acceptable method of promoting responses to self- third is of fundamental importance but even a antigens sufficient to cause immunocontraception. preliminary answer is not possible at present. Jackson et al. (2001) attempted to overcome the problem of lack of immune response to self-antigens in the absence of adjuvant by inserting the cytokine Target species interleukin-4 into mousepox virus with the intention of Population control of native and exotic mammals is increasing the humoral response. The virus was then generally justified by environmental degradation, inserted into the mice with the unwelcome outcome that competition with and predation on native wildlife, the mice all died very quickly. This work caused alarm conflicts with humans over food production, potential because of the possibility that this technology could lead spread of pathogenic infectious diseases and the to a method for simple conversion of relatively possibility of population crashes of over-abundant fauna innocuous viruses into lethal ones, which could be or of wildlife populations near urban areas. Although still used in biological warfare (Finkel 2001). in its infancy, immunocontraception is regarded as being Another possible problem with virus-vectored more humane than the traditional methods of wildlife immunocontraception is the potential for the hori- population control, such as shooting, trapping, zontal transfer of the immunocontraceptive gene into poisoning, or pathogenic agents and its use has strong viruses affecting other species (Becker 2000). While it support from influential animal welfare agencies world- may be possible to create genetically modified wide (Oogjes 1997, Grandy & Rutberg 2002). Table 1 lists organisms without adverse effects on the target mammalian species for which immunocontraception is animals, the effects they might have on related species being investigated and for which the method could be they come in contact with make any use of this applied. In all these species, there are at present no approach questionable. completely efficacious, cost-effective, or socially accep- table methods for population regulation available. Variation in response and genetic change Variation in response to biocontrol agents is a widespread Immune responses to self-antigens phenomenon. This variation has frequently led to Responses to self-antigens are unusual and mainly weak. evolution of a degree of resistance so that the agent is no This constitutes a major barrier to the development of an longer useful. The evolution of resistance to insecticides immunocontraceptive. Table 2 summarizes data on hasbeenreviewedbyMcKenzie (1996). He draws a attempts to induce immunocontraception using a variety distinction between biocontrol agents with responses of antigens in 14 mammalian species. The data in Table 2 within the phenotypic range and those with responses show that in most cases a significant proportion of the outside the phenotypic range of the target species. An population is not contracepted by administration of the agent which initially kills all members of the target species immunocontraceptive antigen. The reason for this could is acting outside the normal phenotypical range, while one be either genetic or environmental. In either case it which kills onlya fraction of the population is acting within indicates that a fraction of the population will continue to the normal phenotypical range. He points out that when breed despite the administration of the contraceptive. In resistance appears in the former case it is frequently most cases, there is likely to be at least in part genetic monogenic, while in the latter case a number of different causes underlying lack of response. If so, the genes for genetic regions are involved, i.e. it is probably multi- lack of response will be selected for and in a compara- factorial. The basic genetic parameter to be estimated in tively small number of generations most of the population either case is heritability, i.e. the extent to which genetic will be non-responsive. This implies that the immuno- variation is controlled by genetic as opposed to environ- contraceptive can be useful for only a short period of time. mental factors. The relative
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